TweetEffect is a free web service which gives detailed information on your Twitter followers. With this service you can find out which of your Twitter updates made people follow or leave you. Unlike other web services based on Twitter, TweetEffect does not require your Twitter password to get the info on followers. It just require you Twitter ID and you can even check the follower details of your friends as well.
Once you enter the Twitter ID, it analyzes the information about your followers. It analyzes your last 200 tweets for changes in followers. Tweets that affected your follower numbers are color coded. Green tweets mean you gained followers and Red Tweets mean you lost followers (or check the arrow icons).
It also gives a overall update on how many followers you lost or gained. Although the service looks good, we are not sure how accurate the service is, but the best part is that it does not require any twitter password for generating data.